The present invention relates to an exhaust gas recirculation system for an engine with a turbocharger.
Recirculation of exhaust gas into the intake gas of an internal combustion engine is widely recognized as a significant method for reducing the production of nitrous oxides (NOx) during the combustion process. The recirculated exhaust gas partially quenches the combustion process and lowers the peak temperature produced during combustion. Since NOx formation is related to peak temperature, recirculation of exhaust gas reduces the amount of NOx formed.
In a normally aspirated internal combustion engine, the intake gas is usually at a lower pressure than the exhaust gas. Under this condition, the introduction of exhaust gas into the intake gas can be performed without difficulty.
However, in a turbocharged internal combustion engine, the pressure relationship between intake and exhaust gasses is often reversed. In a conventional manner, the turbocharger accepts exhaust gas from the engine and using energy from the exhaust gas stream, produces power in a turbine which is used to drive a compressor. The compressor increases the pressure of intake gas being accepted by the engine. The pressure of the intake gas at the compressor discharge, or compressor outlet, is variable, and is related to, among other things, the amount of power being provided by the turbine. The power delivered by the turbine depends upon the temperature and pressure of the exhaust gas, as well as other factors.
Under some operating conditions, such as a diesel engine producing peak torque, the pressure of the intake gas is higher than the pressure of the exhaust gas. Because of this adverse pressure gradient, exhaust gas does not recirculate into the intake unaided.
Various systems have been proposed to provide recirculated exhaust gas for an engine with a turbocharger. In one design, a butterfly-type valve is placed at the exhaust of the turbine so as to backpressure the exhaust system. However, this backpressuring of the turbine results in a thrust load on the turbocharger rotor system which increases wear of the turbocharger bearings. Furthermore, exhaust gas under pressure may leak past the seals and bearings of the turbocharger and flow with the returned lubricating oil into the crankcase of the engine, undesirably increasing engine crankcase pressure.
In yet another design, a combination backpressure/EGR valve is provided at the turbine inlet. In this valve, the actuation of the backpressure valve is mechanically locked to the actuation of the EGR valve. This valve does not include flexibility to change the backpressuring of the exhaust system independently of the recirculation of exhaust gas. This lack of flexibility means that the amount of exhaust gas recirculated may be optimized for a single or narrow range of conditions, and would be non-optimum for most conditions. Also, such a combination valve does not permit backpressuring of the exhaust system during cold start up of the engine without also permitting a high degree of recirculated exhaust gas flow. Such a combination valve would be less than optimum for reduction of white smoke from a diesel engine during cold starting and warm-up.
In another approach, U.S. Pat. No. 5,533,487 discloses an EGR system that attempts to take advantage of both static and dynamic pressure in the EGR system. However, the system relies on specially designed exhaust and inlet ductwork for effective functioning of the system which limits its applicability and flexibility.
A need remains for an exhaust gas recirculation system that provides improvements to current systems and is readily adaptable to existing engine systems.
The present invention provides a total pressure exhaust gas recirculation duct. The system includes a turbocharger for increasing the pressure of the intake gas to the engine. The turbocharger has a turbine inlet scroll that includes a port for diverting a portion of the exhaust gas to the engine intake gas. The diverted exhaust gas from the turbine inlet scroll is conducted to the engine""s intake system through an exhaust gas recirculation duct attached to the scroll. The exhaust gas recirculation duct has an inlet juxtaposed to the port in the inlet scroll. A scoop valve assembly is disposed between the inlet scroll port and the exhaust gas recirculation duct and operates to either prevent or allow exhaust gas flow from the turbine inlet scroll to the exhaust gas recirculation duct. The system may also include an exhaust gas recirculation valve to meter the flow of exhaust gas into the intake gas stream.
In a preferred embodiment, the scoop valve assembly includes an elongated pivot shaft disposed between the scroll and the inlet duct and having at least one free end extending from the scroll and exhaust gas recirculation gas duct assembly. A valve member is attached to the pivot shaft and rotates with the shaft. The valve member has a blade element that is receivable in the port in the inlet scroll. In its open position the valve blade element extends into the exhaust gas stream and diverts a portion of the exhaust gas into the exhaust gas recirculation duct. In a closed position, the valve member seals against the inlet to the exhaust gas recirculation duct thus preventing entry of any of the exhaust gas. A cap and seal can be installed on the pivot shaft""s free end. In a most preferred embodiment, the valve assembly also includes high temperature sleeves to insulate the pivot shaft.
In another version of the invention, the scoop valve assembly includes a blade element positioned in the inlet scroll port and configured to seal the inlet of the exhaust gas recirculation duct when the valve is in a closed position. A valve actuating rod has a distal end attached to the blade element and a proximal end that extends through a passage way providing in either the inlet scroll or the exhaust gas recirculation duct. The actuation rod is slidably movable in the passageway to move the blade element between the open and closed positions. In this embodiment, the passageway preferably includes a guide for the actuating rod and a seal to prevent leakage of the exhaust gas from the passageway.
In yet another version of the invention, there is provided an exhaust gas recirculation system including a turbocharger having an inlet scroll, an exhaust gas recirculation duct attached to the scroll, and a scoop valve assembly between the scroll and duct operable to allow or inhibit exhaust gas flow from the scroll. This embodiment of the invention further includes sensors for monitoring the pressure of the intake gas and the exhaust gas. A controller monitors these pressure signals and then actuates the scoop valve assembly to provide exhaust gas for mixing with the intake gas at an appropriate pressure.
In still another embodiment, the exhaust gas recirculation duct is integral with the inlet scroll.
It is an object of the invention to provide an exhaust gas recirculation duct that uses the kinetic energy of the exhaust gas flow to develop a total pressure sufficient for admission of exhaust gas into the intake system through the EGR valve.
It is another object to provide a duct that allows the EGR function and engine back pressure control to be accomplished independently of each other.
These and other objects and advantages of the present invention will be apparent from the following descriptions of the preferred embodiments and drawings.